Method and system for producing a magnetic field signal usable for locating an underground object

ABSTRACT

Transmitted magnetic field signals useable for locating an underground object, and methods and systems for generating the same. The magnetic field signal has desired spectral characteristics. More specifically, the transmitted magnetic field signal includes a carrier component useable for locating an underground object. The carrier component has a carrier component frequency substantially equal to an integer multiple of 300 Hz. This guarantees that the carrier component frequency is substantially equal to an integer multiple of both 50 Hz and 60 Hz. Such a carrier component allows use of maximum information sidebands in environments that often include harmonically derived interference signals at regular 50 Hz (±0.1 Hz) or 60 Hz (±0.1 Hz) intervals caused by power lines. The transmitted magnetic field signal may also include at least one information sideband including sideband energy. A substantial portion of the sideband energy is contained between the carrier component frequency and a frequency spaced 50 Hz from the carrier component frequency.

FIELD OF THE INVENTION

The present invention is directed to a method and system for generatinga magnetic field signal usable for locating an underground object, suchas an underground boring tool. The present invention is also directed toa magnetic field signal having specific carrier component andinformation sideband characteristics.

BACKGROUND OF THE INVENTION

Several guided and unguided sub-surface (i.e., underground) boring toolsare currently on the market. Guided tools require substantiallycontinuous location and orientation monitoring to provide necessarysteering information. To monitor such an underground tool it isnecessary to track the sub-surface location of the tool. Only once thelocation of the tool is located can a proper depth measurement beobtained, for example, from a measuring position directly above the headof the boring tool which houses a transmitter. Unguided tools would alsobenefit from periodic locating or substantially continuous monitoring,for example, in prevention of significant deviation from planned toolpathways and close tool approaches to utilities or other sub-surfaceobstructions.

One method for locating such sub-surface boring tools includes mountinga magnetic field source on the boring tool and detecting the magneticfield from that field source. This field source can be, for example, asolenoid, or any equivalent transponder capable of generating themagnetic field. When alternating current flows through the solenoid abipolar magnetic field is thereby generated, which can be detected atthe surface by a monitoring device. A vertical component of the magneticfield at the surface will change direction when the monitoring device isdirectly above the solenoid, assuming the solenoid is horizontal.Therefore by noting the position in which that component of the fieldreverses, the position of the solenoid in a horizontal plane can bedetermined. If this is done continuously, the movement of the boringtool on which the solenoid is mounted can be tracked. The depth of thesolenoid can also be gauged by measuring the attenuation of the field atthe surface. This requires the field strength at the solenoid to beknown.

As described above, determinations of the location of underground boringtools rely upon magnetic field measurements. Thus, the reliability andaccuracy of such location determinations can be adversely affected whenthe magnetic field measurements are corrupted. More specifically, thelocation determinations can be adversely affected at the monitoringdevice. The primary sources of magnetic field interference in thisenvironment are power distribution networks. Overhead and/or undergroundpower lines of such power distribution networks produce harmonicallyderived interference signals at regular harmonic intervals of theirfundamental frequencies, 50 Hz (±0.1 Hz) or 60 Hz (±0.1 Hz), through towell above 10 kHz. Besides adversely affecting the reliability andaccuracy of location determinations, magnetic field interference cancause instability in location determinations calculated by themonitoring device, thereby causing a location display to appear unstableto an observer (i.e., user of the monitoring device). Accordingly, thereis a need to reduce the effects of such interference, to thereby improvethe reliability and accuracy of location determinations, and thestability of a display of the location.

It is often useful to know more than just the location of a boring tool.For example, it is often useful to know the orientation (e.g., yaw,pitch and/or roll) of the tool. To provide this information, themagnetic field generated (e.g., by the underground transponder) ismodulated to impart modulated information thereon that can bedemodulated and thus obtained (made available) at the monitoring device.Existing monitoring systems provide limited data throughput (i.e., datatransmission bandwidth from the transponder to the monitoring device),in part due to the need to avoid data corruption by magnetic fieldinterference.

Accordingly, there is a need to increase the data throughput that can beachieved in an environment that includes the interference describedabove.

BRIEF SUMMARY OF THE INVENTION

The present invention relates methods and systems for transmittingmagnetic field signals that are useable for locating an undergroundobject. The present invention also relates to transmitted magnetic fieldsignals having desired spectral characteristics. More specifically, inan embodiment of the present invention, a transmitted magnetic fieldsignal includes a carrier component useable for locating an undergroundobject, where the carrier component has a carrier component frequencysubstantially equal to an integer multiple of 300 Hz.

This guarantees that the carrier component frequency is substantiallyequal to an integer multiple of both 50 Hz and 60 Hz. Such a carriercomponent allows use of maximum information sidebands in environmentsthat often include harmonically derived interference signals at regular50 Hz (±0.1 Hz) or 60 Hz (±0.1 Hz) intervals caused by power lines.

In an embodiment of the present invention, the transmitted magneticfield signal includes an information sideband including sideband energy,where a substantial portion of the sideband energy is contained betweenthe carrier component frequency and a frequency spaced 50 Hz from thecarrier component frequency. The substantial portion includes, forexample, at least 50% of the sideband energy.

In an embodiment of the present invention, the transmitted magneticfield signal includes a lower information sideband and un upperinformation sideband. The lower information sideband includes lowersideband energy, where a substantial portion of the lower sidebandenergy is contained between the carrier component frequency and afrequency spaced 50 Hz below the carrier component frequency. Similarly,the upper information sideband includes upper sideband energy, where asubstantial portion of the upper sideband energy is contained betweenthe carrier component frequency and a frequency spaced 50 Hz above thecarrier component frequency.

The lower and upper information sidebands are, according to anembodiment of the present invention, amplitude modulation sidebands thatconvey data. Additionally, the lower and upper information sidebands arepreferably symmetric about the carrier component. In one embodiment, thelower and upper information sidebands convey between 50 and 80 bits persecond of information, with 75 bits pers second being a convenient bitrate.

An embodiment of the present invention is directed to a method forgenerating a magnetic field signal usable for locating an undergroundobject. The method includes the step of producing a drive signalincluding a carrier component having a carrier component frequencysubstantially equal to an integer multiple of 300 Hz (e.g., within ±3 Hzof an integer multiple of 300 Hz). The drive signal is then used todrive a transponder to generate a magnetic field signal having amagnetic field carrier component equal to the carrier componentfrequency. The drive signal can be produced by generating a referencesignal having a reference signal frequency substantially equal to theinteger multiple of 300 Hz. An information signal is encoded to producean encoded information signal. The reference signal is then modulatedwith the information signal at a predetermined bit rate to produce thedrive signal. The drive signal includes the carrier component and atleast one information sideband including sideband energy. The encodingand bit rate cause a substantial portion of the sideband energy to becontained between the carrier component frequency and a frequency spaced50 Hz from the carrier component frequency. Similarly, the transmittedsignal, produced by driving the transponder with the drive signal,includes a magnetic field signal carrier component equal to the carriercomponent frequency and at least one magnetic field signal informationsideband including magnetic field signal sideband energy. A substantialportion of the magnetic field signal sideband energy is between thecarrier component frequency and the frequency spaced 50 Hz from thecarrier component frequency.

Another embodiment of the present invention is directed to a system forgenerating a magnetic field signal usable for locating an undergroundobject. The system includes a subsystem to produce a drive signalincluding a carrier component having a carrier component frequencysubstantially equal to an integer multiple of 300 Hz. The system alsoincludes a transponder (e.g., a solenoid) to generate the magnetic fieldsignal when driven by the drive signal. The generated magnetic fieldsignal has a magnetic field carrier component equal to the carriercomponent frequency. If the drive signal is a modulated signal, then themagnetic field signal also includes one or more information sidebands.In an embodiment of the present invention, the subsystem includes areference signal generator (e.g., an oscillator) to produce a referencesignal having a reference signal frequency substantially equal to theinteger multiple of 300 Hz. The subsystem may also include one or moresensors that produce an information signal. An encoder encodes theinformation signal to produce an encoded information signal. A modulatormodulates the reference signal with the encoded information signal at apredetermined bit rate to produce the drive signal. The drive signalincludes the carrier component and at least one information sidebandincluding sideband energy. The encoder and bit rate cause a substantialportion of the sideband energy to be contained between the carriercomponent frequency and a frequency spaced 50 Hz from the carriercomponent frequency.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The present invention is described with reference to the accompanyingdrawings. In the drawings, like reference numbers indicate identical orfunctionally similar elements. Additionally, the left-most digit(s) of areference number identifies the drawing in which the reference numberfirst appears.

FIG. 1 illustrates an exemplary environment in which embodiments of thepresent invention are useful;

FIG. 2A shows an underground solenoid, viewed along the axis of thesolenoid, and first and second measuring points;

FIG. 2B is a side view of the same solenoid and measuring points of FIG.2A;

FIG. 3 is a graph showing the ratio of the axial component of themagnetic field at a measuring point to the radial component as afunction of the angle between the axis of the solenoid and a line fromthe measuring point to the center of the solenoid;

FIG. 4 is an illustration useful for showing how a sub-surface boringtool can be located;

FIG. 5 illustrates an exemplary transmit device in which specificembodiments of the present invention can be used;

FIG. 6 is an exemplary frequency domain plot of a magnetic field signalproduced by the transmit device of FIG. 5 (e.g., using Amplitude ShiftKeying or On-Off Keying modulation);

FIG. 7 illustrates an exemplary receive/monitoring device in whichspecific embodiment of the present invention can be used;

FIG. 8 illustrates additional details of the digital signal processor(DSP) stage/module of FIG. 7;

FIG. 9A is an exemplary frequency domain plot of an interference signal(e.g., produced by an overhead or underground power line in the UnitedStates);

FIG. 9B is an exemplary frequency domain plot of an interference signal(e.g., produced by an overhead or underground power line in Europe);

FIG. 9C is an exemplary frequency domain plot that includes aninterference signal (in solid line) that is in close proximity with acarrier signal frequency (in dashed line) (e.g., for interferenceproduced by power lines in the United State or Europe);

FIG. 10 is flow diagram illustrating a method for generating a magneticfield signal, according to an embodiment of the present invention;

FIG. 1A illustrates an exemplary received magnetic field signal after itis shifted down to a near DC signal;

FIG. 11B illustrates an exemplary interference harmonic signal after itsfrequency has been shifted down;

FIG. 11C illustrates an exemplary received magnetic field signal havinga beat frequency due to the interference harmonic signal of FIG. 11B.

FIG. 12 is a flow diagram illustrating a method for reducinginterference according to an embodiment of the present invention; and

FIG. 13 is a functional block diagram of a system for reducinginterference according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Prior to explaining specific details of the present invention, anexemplary environment where the present invention is useful will firstbe described. Referring to FIG. 1, an underground transmit device 102transmits a magnetic field signal 104 that can be received by areceive/monitoring device 110. Transmit device 102 is typically mountedto, or integrally formed with, a boring/drilling tool (not shown in FIG.1). Receive device 110 is typically part of, or in communications with,a monitoring device that monitors, among other things, a location oftransmit device 102 (and thereby, a location of the boring tool). Aswill be explained below, receive device 110 determines the location oftransmit device 102 based on a received magnetic field signal (includingmagnetic field signal 104 generated by transmit device 102). As willalso be explained in more detail below, the monitoring device candetermine additional information, when magnetic field signal 104 ismodulated to convey such additional information.

In addition to receiving magnetic field signal 104, receive device 110may also receive magnetic interference signals 106 a and/or 106 b, thatmay be produced by overhead and/or underground power lines (not shown).Additional details of such interference signal are discussed below.

Referring now to FIGS. 2A and 2B, transmit device 102 typically includesa solenoid 210 or other equivalent transponder that is driven to radiatemagnetic field signal 104. An alternating electric current having aknown magnitude is passed through the coils of solenoid 210. The flow ofelectric current through solenoid 210 generates a magnetic field (i.e.,magnetic field signal 104). The carrier component of magnetic fieldstrength at solenoid 210 itself can be measured before solenoid 210 isburied. As shown in FIG. 2B, solenoid 210 is arranged so that its axis211 is horizontal. Also shown in FIGS. 2A and 2B are a first measuringpoint 220 and a second measuring point 221 at which magnetic fieldmeasurements are made above ground. Measuring points 220 and 221 aretypically antennas of an antenna array (e.g., an antenna array 702,shown in FIG. 7). The antenna array is part of receive device 110.

These measuring points 220 and 221 lie in a vertical plane containingaxis 211 of solenoid 210. Measuring points 220 and 221 as shown arepositioned one above the other at a known vertical separation. Thelocation of first measuring point 220 relative to solenoid 210 can bedefined by a distance r₁, between them and the angle θ₁ above thehorizontal of solenoid 210 to first measuring point 220. The relativelocations of second measuring point 221 and solenoid 210 are similarlydefined by distance r₂ and angle θ₂.

A horizontal component f_(h) and a vertical component f_(v) of the fieldstrength are measured at first measuring point 220, and a ratio f_(h)/f_(v) is calculated by a system computer (e.g., a system computer 720,shown in FIG. 7). The ratio f_(h)/f_(v) is a function of angle θ₁, andthis function can be determined analytically and stored in a memory (notshown) of the system computer. The function is shown graphically in FIG.3. It can be seen from FIG. 3 that the function is monotonic, that is,each value of the ratio f_(h)/f_(v) corresponds to one and only onevalue of angle θ₁. Therefore when the calculated value of the ratiof_(h)/f_(v) is input to the system computer, the value of θ₁ can bederived.

Similar measurements taken at the second measurement point 221 enableangle θ₂ to be derived in the same way. Once these angles have beendetermined the distance between solenoid 210 and measuring points 220and 221 could be derived by triangulation. Instead the absolute valuesof the field strength at measuring points 220 and 221 are measured, andsince the field strength at the solenoid 210 itself is known this givesthe attenuation of the field at measuring points 220 and 221. From theattenuation of the field, the distances r₁, r₂ between solenoid 210 andmeasuring points 220 and 221 can be calculated by a system computer(e.g., system computer 720, shown in FIG. 7). The attenuation variesboth with the distances between the measuring points and the solenoid,and also the angles θ₁ and θ₂. However the angles θ₁ and θ₂ are known,and therefore the attenuation can be determined using, for example, alook-up table that relates attenuation and angle. Once the distances r₁,r₂ and angles θ₁, θ₂ have been determined, the location of solenoid 210relative to measuring points 220 and 221 is now known. By averaging theresults from the two measuring points 220 and 221 reliability can beincreased and the effects of noise can be decreased.

In the above analysis the solenoid was horizontal and therefore thehorizontal and vertical components f_(h), f_(v) correspond to the axialand radial components of the magnetic field, respectively. If thesolenoid was not horizontal, then either the axial and radial componentscould be measured directly, or the vertical and horizontal componentsmeasured and then a correction made to determine the axial and radialcomponents. Additional details of a method for locating an undergroundsolenoid (and thus, an underground drilling tool) in the mannerdescribed above are disclosed in U.S. Pat. No. 5,917,325 to Smith, whichis incorporated herein in its entirety by reference.

FIG. 4 shows the application of this principle to locating anunderground boring tool 430. Solenoid 210 is mounted on boring tool 430such that the axis of solenoid 210 is parallel to the normal motion ofboring tool 430. In this way, as boring tool 430 burrows through theearth, solenoid 210 moves with boring tool 430 and continuousmeasurements of the type described above can be used to track the motionof boring tool 430. Receive device 110 (or multiple received devices110) are located above ground. On the basis of the measurements of itslocation, boring tool 430 is controlled using conventional controlmethods.

It is often useful to know more than just the location of boring tool430. For example, it is often useful to know the orientation (e.g., yaw,pitch and/or roll) of tool 430. To provide this information, themagnetic field generated by solenoid 210 is modulated to impartmodulated information thereto that can be demodulated and thus obtainedat a monitoring device (e.g., the same monitoring device used to trackthe location of underground boring tool 430).

Exemplary Transmit Device

FIG. 5 illustrates exemplary details of transmit device 102 that can beused to transmit a modulated magnetic field signal. Solenoid 210 can beconsidered part of transmit device 102. Exemplary transmit device 102includes a pitch sensor 502, a roll sensor 504 and a temperature sensor506, all of which are known in the art. Digital output signals fromsensors 502, 504 and 506 are multiplexed by a transmit system computer510 (or by a multiplexer) to produce an information signal 512 providedto an encoder 520. Encoder 520 encodes information signal 512 to producean encoded information signal 522. Additional details of an encodingscheme employed by encoder 520, according to embodiments of the presentinvention, are discussed below. A reference signal generator 540 (e.g.,a local oscillator) produces a reference signal 542 that has a referencefrequency. A modulator 530 modulates reference signal 542 with encodedinformation signal 530 at a predetermined bit rate (also referred to asa modulation rate) to produce a transponder drive signal 532.Accordingly, the above mentioned elements of transmit device 102 can beconsidered a subsystem 550 that produces drive signal 532.

Drive signal 532 includes a carrier component, having a frequency equalto the reference frequency, and at least one information sidebandincluding sideband energy (an information sideband is also referred toas a modulation sideband because it arises as a result of modulatingreference signal 542). Accordingly, the frequency of the carriercomponent is controlled by reference signal generator 542. The spectralshape (i.e., frequency spectrum) of the sideband(s) is a function of thebit rate and the encoding scheme of encoder 520, as will be explained inmore detail below. Modulator 530 can perform amplitude modulation, suchas Amplitude Shift Keying (ASK) or On-Off Keying (OOK), as would beapparent to one of ordinary skill in the art, to produce upper an lowerinformation sidebands. Single sideband modulation schemes canalternatively be used, as would be apparent to one of ordinary skill inthe art.

Drive signal 532 drives a transponder, such as solenoid 210, to therebyproduce magnetic field signal 104. FIG. 6 is an exemplary frequencydomain plot of magnetic field signal 104 produced by undergroundtransmit device 102 (using ASK or OOK modulation). Magnetic field signal104 includes a narrow band carrier component 602 at a carrier componentfrequency F_(c) and upper and lower modulation sidebands. As justmentioned, the spectral shape of the upper information side band (USB)and the lower information side band (LSB) are functions of the encodingscheme and bit rate. This is described in more detail below.

Exemplary Receive Device

FIG. 7 illustrates exemplary details of receive device 106. Receivedevice 106 receives magnetic field signals (e.g., magnetic field signal104) from transmit device 102 and determines locations based on carriercomponents of the signals. Exemplary receive device 106 includes anantenna array 702 (e.g., including antennas at locations 220 and 221),and a frequency down-converter, amplifier, and filter stage/module 704.Exemplary receive device 106 also includes an analog to digitalconverter (A/D) 706, a digital signal processor (DSP) 708, a receivesystem computer 720 (e.g., a microcontroller or microprocessor) and adisplay 730. Antenna array 702 detects magnetic field signal 104, andprovides an RF signal 703 representative of the magnetic field signal tothe next stage/module (signal conditioner 704). RF signal 703 includesan RF carrier component an at least one information sidebandcorresponding to the magnetic field signal. In an exemplary arrangement,RF signal 703 is frequency down-converted (e.g., shifted from about 8400kHz down to about 6 Hz), amplified and filtered at stage/module 704,before being converted a digital signal by A/D 706. The resultingdigital signal is then fed to DSP 708, which performs digital signalprocessing on the digital signal received from A/D 706. An output of DSP708 is provided to receive system computer 720. System computer 720performs the necessary calculations to generate a location output (e.g.,using the process explained above) to be displayed on display 730.Display 730 is, for example, a liquid crystal display (LCD). Display 730can include, for example, a compass graphic, forward and backwardindicator arrows, and left and right indicator arrows.

Another output from frequency down-converter, amplifier, and filterstage/module 704 is provided to a data recovery stage/module 712, whichperforms, among other things, demodulation of the at least oneinformation/modulation sideband. An output of data recovery stage/module712 is provided to a decoder 714 which decodes encoded information inthe information sideband(s) of signal 104/703. The decoded data is thenprovided to system computer 720, which generates additional outputs tobe displayed on display 730 (e.g, temperature, pitch, etc.).

Exemplary details of DSP 708 are shown in FIG. 8. Exemplary DSP 708includes a digital down converter 802 that mixes-down the digital signalreceived from A/D 706 to an almost direct current (DC) digital signal(e.g., having a frequency near zero Hz, and preferably, less than 0.25Hz). The near DC digital signal is then decimated by a decimator 804,which filters and reduces the bit rate of the near DC digital signal(e.g., from 240 samples per second to 30 samples per second). Decimator804 can include, for example, one or more finite impulse response (FIR)filters and/or one or more infinite impulse response (IIR) filters. Ifthe digital signal received from A/D 706 includes inphase (I) andquadrature (Q) components (e.g., produced by a quadrature mixer ofstage/module 704), then a rectangular-to-polar stage/module 806 isincluded to produce magnitude and phase components. DSP 708 providessamples to system computer 720 that are representative of detectedmagnetic field strength of the carrier component of the magnetic fieldsignal. System computer 720 tracks the location of an undergroundtransponder (e.g., solenoid 210) based on these sample (e.g., using theprocess described above).

Overview of the Present Invention

In the above described environment, determinations of the location ofsolenoid 210 (and thus boring tool 430) rely upon magnetic fieldstrength measurements of the carrier component of a detected magneticfield signal. Thus, it is clear that the reliability and accuracy ofsuch location determinations can be adversely affected when the magneticfield measurements are corrupted. Stated another way, the locationdeterminations can be adversely affected if magnetic field strengthmeasurements of magnetic field signal 104 are altered at measuring point220 and/or measuring point 221 due to magnetic field interference.Magnetic field interference can be produced, for example, by overheadpower lines and/or buried power lines. Besides affecting the reliabilityand accuracy of location determinations, interference can causeinstability in location determinations calculated by receive systemcomputer 720. An effect of this is that the solenoid location, asindicated on display 730, is unstable. For example, the compass graphicand/or left/right indicator arrows may appear unstable to an observer ofdisplay 730.

As mentioned above, the primary sources of magnetic field interferenceare power distribution networks that produce harmonically derivedinterference signals at regular 50 Hz (±0.1 Hz) or 60 Hz (±0.1 Hz)intervals from their fundamental frequency through to well above 10 kHz.More specifically, it is typically overhead power lines and/or buriedpower lines of a power distribution networks that produce such magneticfield interference. For example, power lines in the United Statesgenerally produce frequency comb lines at approximately 60 Hz intervals,as shown in FIG. 9A. Power lines in Europe generally produce frequencycomb lines at approximately 50 Hz intervals, as shown in FIG. 9B. Powerlines in other areas of the world typically produce interference signalsthat are similar to those in either Europe or the United States.

Embodiments of the present invention are directed to reducing (andpreferably cancelling) such interference, to thereby improve thereliability and accuracy of location determinations, and the stabilityof a display of the location (e.g., on display 730).

Further embodiments of the present invention are directed to increasing(and preferably maximizing) the data throughput (i.e., of transmitdevice 102) that can be achieved in an environment that includes theinterference frequency harmonics described above (also referred to asfrequency comb lines and harmonically derived interference).

Carrier Component Frequency

A system for monitoring the location of underground objects (e.g.,boring tool 430) is preferably designed such that it can operate inenvironments including either of the above-mentioned harmonicallyderived interference (i.e., interference at approximately 50 Hzintervals or interference at approximately 60 Hz intervals). This is sothe same system can be successfully used in different areas of theworld. Accordingly, one methodology for attempting to avoid magneticfield interference is to use a carrier component frequency that ispurposely offset from both (i.e., avoids) the 50 Hz and 60 Hz harmonicintervals by at least a predetermined frequency offset (e.g., at least 8Hz). For example, U.S. Pat. No. 5,767,678, entitled “Position andOrientation Locator/Monitor,” uses a carrier component frequency of32768 Hz, which is between the 546^(th) and 547^(th) harmonics of 60 Hz,and between the 655^(th) and 656^(th) harmonics of 50 Hz. A limitationof such a system is that any information sidebands of the carrier signal(produced by modulating a carrier reference signal) must be sufficientlynarrow to avoid overlapping the 50 Hz or 60 Hz comb lines in order toavoid interference. In other words, the bandwidth of the informationsidebands becomes limited, at as a result, the data throughput of such alow bandwidth system is correspondingly limited.

The present invention uses a carrier component frequency that is counterintuitive. Rather than using a carrier component frequency that isoffset from 50 Hz and 60 Hz harmonic intervals by at least apredetermined frequency offset to avoid interference (as suggestedabove), the carrier component frequency is preferably selected such thatit is substantially equal to (e.g., ±3 Hz) an integer multiple of both50 Hz and 60 Hz. More specifically, the selected carrier componentfrequency is substantially equal to an integer multiple of 300 Hz. Thissignificantly increases the probability that a carrier component of atransmitted magnetic field signal (e.g., signal 104) will be affected bymagnetic field interference when power lines are within the vicinity oftransmit device 102 and/or receive device 110. Thus, selection of such acarrier component frequency is counter intuitive. Embodiments of thepresent invention are directed to reducing (and preferably cancelling)such interference near the carrier component frequency, as will beexplained in detail below. A benefit of using a carrier componentfrequency that is substantially equal to a common multiple of both 50 Hzand 60 Hz, is that the width of information sidebands (produced bymodulating the carrier component frequency with an encoded informationsignal) can be increased, thereby increasing data throughput. This isshown in FIG. 9C.

Referring back to FIG. 5, the frequency spectral characteristics oftransponder drive signal 532 produced by modulator 530 (and similarly,the frequency spectral characteristics of magnetic field signal 104transmitted by solenoid 210) is a function of a reference frequency(e.g., generated by reference signal generator 540), modulation scheme,bit rate, and the encoding scheme of encoder 520. More specifically, acarrier component of drive signal 532 (and magnetic field signal 104)has a carrier component frequency that is equal to the frequency ofreference signal 542. The modulation scheme creates or provides theinformation/modulation side band(s). For example, the modulation schemecontrols whether the magnetic field signal 104 is a single sideband or adouble sideband signal. Amplitude Shift Keying (ASK) and On-Off Keying(OOK) modulations schemes, for example, produce a double sidebandsignal, as shown in FIG. 6 and 9C. The bit rate and the encoding schemecontrol the bandwidth of the sideband(s).

An embodiment of the present invention is directed to an encoding schemethat increases (and preferably maximizes) use of the availableinformation sideband bandwidths, as will be explained in detail below.An embodiment of the present invention is more generally directed tonestling the information sideband(s) between interference comb lines tothereby both increase (and preferably maximize) data throughput andreduce (and preferably minimize) the probability of interferenceaffecting the information sideband(s) (and thus, the data).

The increased bandwidth in which information sidebands can be nestled isa result of transmitting magnetic field signal including a carriercomponent frequency in close proximity to an interference harmonic, asdescribed above. Accordingly, an embodiment of the present invention isdirected to a transmitted magnetic field signal including a carriercomponent usable for locating an underground object, where the carriercomponent has a carrier component frequency substantially equal to aninteger multiple of 300 Hz. An embodiment of the present invention isalso directed to such a magnetic field signal also including one or moreinformation sidebands each including sideband energy, where asubstantial portion (e.g., at least 50%) of each sideband energy iscontained between the carrier component frequency and a frequency spaced50 Hz from the carrier component frequency. As mentioned above, theinformation sideband(s) can be amplitude modulation sidebands. If thereare lower and upper information sidebands, as shown in FIG. 6 and 9C,the lower and upper information sidebands are preferably symmetric aboutthe carrier component. This maximizes the bandwidth of each of thesidebands. It is the information sidebands that convey data fromtransmit device 102 to receive device 110. More specifically, thesideband(s) may convey data at a bit rate between 50 and 80 bits persecond, with 75 bits pers second being convenient because it is a baudrate that is often supported by available microprocessors used insub-surface tool monitoring system. It is noted that there may beadditional sidebands that are further away from the carrier componentfrequency. These additional sidebands are harmonically related to thesideband(s) that are closest to the carrier component frequency.However, these additional sidebands have much less energy than thesidebands closest to the carrier component frequency.

A method 1000 for generating a magnetic field signal usable for locatingan underground object, according to an embodiment of the presentinvention, is described with reference to FIG. 10. In the description ofmethod 1000, reference is made back to FIG. 5 where appropriate.

At a first step 1002, a reference signal having a reference signalfrequency substantially equal to the integer multiple of 300 Hz isgenerated, for example, by reference signal generator 540. At a nextstep, 1004, an information signal is received, for example, from asystem computer 510 or multiplexer. The order of these steps is notmeant to be limiting.

At a next step, 1006, the information signal is encoded to produce anencoded information signal, for example, by encoder 520. Next, at a step1008, the reference signal is modulated with the encoded informationsignal at a predetermined bit rate (also know as modulation rate) toproduce the drive signal. The drive signal includes a carrier componentand at least one, and probably two, information sidebands includingsideband energy. The encoding of the present invention and choice of bitrate cause a substantial portion of the sideband energy to be containedbetween the carrier component frequency and a frequency spaced 50 Hzfrom the carrier component frequency. The encoding of the presentinvention also causes the carrier component to have a substantiallyconstant average energy. This is important so that the encoding does notaffect location determinations that are based on detected magnetic fieldstrength of the carrier component.

Steps 1002 through 1008 can be collectively thought of as a step 1010 ofproducing a drive signal including a carrier component and at least onefrequency sideband, as shown in dashed line. The carrier component has acarrier component frequency substantially equal to an integer multipleof 300 Hz. The carrier component also has a substantially constantaverage energy due to the encoding scheme of the present invention.

At a next step 1012, a transponder (e.g., solenoid 210) is driven by thedrive signal to generate a magnetic field signal. The magnetic fieldsignal has a magnetic field signal carrier component and at least oneinformation sideband. The magnetic field carrier component has asubstantially constant average energy and a frequency equal to thereference frequency. The at least one magnetic field signal informationsideband includes magnetic field signal sideband energy. A substantialportion of the magnetic field signal sideband energy is containedbetween the carrier component frequency and a frequency spaced 50 Hzfrom the carrier component frequency. As described above, the magnitudeof the carrier component (when detected) is usable for determining alocation of the transponder (and thus any tool in proximity of thetransponder). The information sideband(s) convey modulated information.

As mentioned above, according to an embodiment of the present inventiona carrier component of magnetic field signal 104 (produced by transmitdevice 102) has a carrier component frequency that is substantiallyequal to an integer multiple of 300 Hz, thereby guaranteeing that thecarrier component frequency is substantially equal to an integermultiple of both 50 Hz and 60 Hz. This provides a maximum amount ofspectral room for information sidebands. However, as mentioned above,using a carrier component frequency that is substantially equal to aninteger multiple of 300 Hz will significantly increase the probabilitythat measurements of the carrier component of magnetic field signal 104will be affected by magnetic field interference when power lines arewithin the vicinity of transmit device 102 and/or receive device 110.Location determinations are typically based on magnetic field strengthmeasurements of the carrier component of magnetic field signal 104, forexample, using the process described above in connection with FIGS. 2A,2B and 3. Specific embodiments of the present invention are directed toreducing (and preferably cancelling) magnetic field interference when itis present. However, when using a carrier component frequency that issubstantially equal to an integer multiple of 300 Hz, it would bebeneficial to select a carrier component frequency where interferenceharmonics tend to be relatively low (as compared to other interferenceharmonics). Field tests have revealed that even harmonics of 50 Hz and60 Hz fundamental frequencies generally tend to have less energy contentthan adjacent, odd harmonics in a given frequency region. Accordingly,in an embodiment of the present invention the carrier componentfrequency is substantially equal to an even multiple of both 50 Hz and60 Hz. Two exemplary carrier frequencies that satisfy this requirementare 8400 Hz and 33600 Hz. Specifically, 8400 Hz corresponds to the140^(th) harmonic of 60 Hz, and the 168^(th) harmonic of 50 Hz. 33600 Hzcorresponds to the 560^(th) harmonic 60 Hz and the 672^(nd) harmonic of50 Hz.

Encoding

As just mentioned, an embodiment of the present invention is directed tonestling the information sideband(s) of a magnetic field signal betweeninterference comb lines. This is accomplished by using an appropriatecarrier component frequency, bit rate, modulation scheme, and encodingscheme. The carrier component frequency is preferably selected such thatit is substantially equal to an even integer multiple of both 50 Hz and60 Hz, as stated above. Also, the carrier component of magnetic fieldsignal 104 has a frequency preferably located close to an even harmonicof both 50 Hz and 60 Hz (when power lines are in the vicinity oftransmit device 102 and/or receive device 110). Accordingly, 50 Hz and60 Hz harmonics adjacent to the carrier component are odd harmonics.

In the present invention, information sidebands, as depicted in FIG. 9C,are produced by modulating reference signal 542 with encoded informationsignal 522 at a given bit/modulation rate. Assume, for example, abit/modulation rate of 75 bits per second (bps). If each of the encodedbits in the encoded information signal 522 is a complement of itsneighboring bits (i.e., 1010101010 . . . ), then the highest frequencymodulated signal for that baud rate is produced. This highest frequencymodulated signal established the outer bounds of the modulationsidebands. For example, the outer bounds of the sidebands would beapproximately 37.5 Hz from the center frequency (i.e., 1÷(2÷75)=37.5),as shown in dashed line FIG. 9C.

Now assume that at the maximum number of consecutive identical bits istwo (i.e., 110011001100). This would produce a inner boundary of thesideband that is approximately 18.75 Hz from the center frequency, alsoshown in dashed line FIG. 9C. In practice, the boundaries of thesideband(s) would not be as sharp as those shown in FIG. 9C.Nevertheless, the information of interest is conveyed by modulationsidebands having energy contained substantially within outer boundsequal to 37.5 Hz and inner bounds equal to 18.75 Hz spaced from thecarrier component frequency. Thus, the information of interest issubstantially unaffected by the harmonic interference comb lines. Statedanother way, the information sidebands are spectrally shaped (by choiceof encoding, bit rate, and modulation) such that a substantial portion(e.g., at least 50%) of the sideband energy is contained between thecarrier component frequency and a frequency spaced 50 Hz from thecarrier component frequency.

As just explained, encoding is one of the factors that affects thespectral shaping of the information sidebands. An embodiment of thepresent invention is directed to an encoding scheme (including a codingmethod and apparatus) that can be used to spectrally shape theinformation sidebands such that they are nestled between a centerinterference spectral harmonic and its adjacent spectral harmonics. Anencoding scheme of the present invention is now described. In anembodiment of the present invention, each four (4) bits of data (e.g.,produced by pitch sensor 502, roll sensor 504 and/or temperature sensor506) are encoded into an eight (8) bit code word. There exist sixteen(16) possible combinations of four bits. There exist two hundred andfifty six (256) possible combinations of eight bits (i.e., there are 256different code words that can be created from 8 bits). The 16 codewords, of the possible 256 code words, are chosen such that:

-   -   1) the bits are balanced (i.e., there are an equal number of        logic ones and logic zeros);    -   2) no more than two consecutive bits are identical bits (i.e.,        there are no more than two ones or two zeros in a row); and    -   3) each code word is different from any of the other code words        by at least two bits.

Limiting the number of consecutive identical bits (to two) has theeffect of attenuating low frequency spectral components in modulated(e.g., amplitude modulated) signal 532. The balancing of the bits causesthe carrier component of modulated signal 532 to have a substantiallyconstant average energy. These effects are also translated to magneticfield signal 104. The requirement that each code word is different fromany of the other code words by at least two bits exists so that a singlebit error will not cause one code to be mistaken for another one of thecode words.

The encoded data is transmitted synchronously in frames. Each frameincludes a header word followed by the encoded bits (i.e., followed bycode words). For example, each frame can include an 8 bit headerfollowed by a predetermined number of code words (e.g., 70 code words).Receive device 110 synchronizes itself with the received magnetic fieldsignal using the header words. Accordingly, a header may also bereferred to as a synch code, a synch byte, or a frame marker code.

The header is chosen such that:

-   -   1) it is different from any of the 16 possible code words by at        least two bits; and    -   2) it is different from any contiguous set of 8 coded bits        (e.g., 3 bits of a first code word contiguous with 5 bits of an        adjacent code word) by at least one bit;

The above requirements minimize the chances that a signal bit errorwithin the data will cause a decoder (e.g., decoder 714) to mistake acode word for a header, or vice versa.

In order for a header to meet the above requirements, more than twoconsecutive bits are made to be the same state, and the bits of theheader are not be balanced. However, to minimize the deleterious effectsof the header on a receiver (e.g., receive device 110), only one groupof three bits in a row can exist within a header. Additionally, tocompensate for the unbalanced bits in the header, the two headers ofsuccessive frames are compliments of one another. This improves packetto packet energy balancing and ensures that the number of logic one bitsand logic zero bits is equal over two successive frames, therebymaintaining the substantially average energy of the signal. This alsominimizes the risk of false frame synching by a receiver operating in ahigh noise/interference environment.

A set of code words, according to an embodiment of the presentinvention, are shown below in Table 1. The 16 possible combinations of 4bits are shown in the left column. In the right column are shown 16exemplary 8 bit code words that meet the above mentioned codingrequirements (also referred to as conditions). For each row of the Table1, the 8 bit code word in the right column can be used to represent thecorresponding 4 bits in the left column. However, the order (i.e.,matching of 4 bits to 8 bit code words) shown in the table is arbitrary.That is, an 8 bit code in the right column can be assigned to representany one of the 4 bits in the left column, depending on implementation.

TABLE 1 Possible Combinations of 4 Bits 8 Bit Codes Words 0000 101010100001 10101001 0010 10100110 0011 10100101 0100 10011010 0101 100110010110 10010110 0111 10010101 1000 01101010 1001 01101001 1010 011001101011 01100101 1100 01011010 1101 01011001 1110 01010110 1111 01010101

An important additional advantage of the codes shown in Table 1 is thata single bit error will not cause one code to be mistaken for anotherone of the code words. This is because each code word is different fromany other code words by at least two bits. Accordingly, confidence inreceived decoded data is thereby increased.

There exist additional 8 bit code words (not shown in Table 1) that meetthe above mentioned requirements, i.e., 01001010 and 10110010.Preferably, the 16 codes words (i.e., 8 bit code words) selected torepresent the 16 possible 4 bit combination are selected from the 18possibilities presented (i.e., from the 16 code words shown in Table 1and code words 01001010 and 10110010).

Exemplary headers that meet the above specified requirements are01011101 and 10100010. A one bit error in header 01011101 may produce acode word, for example, code word 01011001. However, this should notpresent much of a problem since a receiver (e.g., receive device 110)will not be looking for data before has been synchronized. Similarly, aone bit error in code word 01011001 may produce a header, for example,header 01011101. However, this should not present much of a problemafter synchronization since the receiver (e.g., receive device 110)knows precisely which frames at which to look for a header. Even priorto synchronization (i.e., during a synchronization process), this shouldnot present a problem.

Assuming a baud rate of 75 bps, use of the above described encoding ofthe present invention results in a magnetic field signal (e.g., magneticfield signal 104) having information sideband(s) within a recoverablebandwidth of ±37.5 Hz from the carrier component frequency. When theselected carrier component frequency is substantially equal to a commonmultiple of both 50 Hz and 60 Hz (e.g., the carrier component frequencyis 8400 Hz), and thus located close to an interference harmonic (whenpower lines are in the vicinity of receive device 110), then theinformation sidebands will be between a center interference spectralharmonic (i.e., close to 8400 Hz in this example) and adjacentinterference harmonics. Continuing with this example, the adjacentinterference harmonics in the United States would be at approximately8340 Hz and 8460 Hz, as shown in FIG. 9C. Thus, the encoding scheme ofthe present invention can be used to spectrally shape the LSB so that itis between 8340 Hz and 8400 Hz, and to spectrally shape the USB so thatit is between 8400 Hz and 8460 Hz. In Europe, the adjacent interferenceharmonics would be at approximately 8350 Hz and 8450 Hz, as shown inFIG. 9C. The same encoding scheme of the present invention would alsospectrally shape the LSB so that it is between 8350 Hz and 8400 Hz, andthe USB so that it is between 8400 and 8450, also shown in FIG. 9C.Thus, the encoding scheme of the present invention can be used inmonitoring devices employed throughout the world.

As just described, embodiments of the present invention are directed tomethods for encoding. More specifically, an embodiment of the presentinvention is directed to a method for producing an encoded informationsignal having attenuated low frequency spectral components and asubstantially constant average energy. Generally, the method includesproducing an M bit code word for each N bits of the information signalaccording to the following conditions: (a) each code word includes anequal number of logic zero bits and logic one bits; (b) each code wordincludes no more than two consecutive identical bits; and (c) M isgreater than N. As described above, in a preferred embodiment M equalseight and N equals four, and the generating of the code words includesproducing an eight bit code for each four bits of the informationsignal. The resulting encoded information signal can be used to producea magnetic field signal having the desired spectral characteristicsdiscussed above. However, in alternative embodiments of the presentinvention M and N are not limited to these preferred values (i.e., 8 and4, respectively), as other values for M and N can be used to produce thedesired spectral characteristics. Although not preferred, the conditionthat no more than two bits are identical can be modified if additionallow frequency components can be tolerated (for example, the conditioncan be that no more than three consecutive bits are identical). One ofordinary skill in the art will appreciate that the number of acceptableconsecutive bits can be increase if the bit rate is increasedaccordingly.

As described above, an embodiment of the present invention also includesgenerating frames having an equal number of logic zero bits and logicone bits in each pair of consecutive frames. This is accomplished byproducing a header for each predetermined number of codes words (e.g.,70 code words) according to the following conditions: (i) each header isdifferent from any of the generated code words by at least two bits; and(ii) each header is a compliment of any immediately preceding header andany immediately following header. As mentioned above, each header ispreferably different from any possible contiguous bits of any possiblepair of consecutive code words by at least one bit. The headers ispreferably the same length as the code words.

Referring back to FIG. 10, the encoding schemes of the present inventioncan be used at step 1006 to produce a magnetic field signal having thedesired frequency spectral characteristics. Embodiments of the presentinvention are also directed to encoder 520. Encoder 520 includes, forexample, code lookup table or memory that stores the plurality ofdifferent codes words each representative of a different combination ofbits. The encoder may also store at least a first header and a secondheader, where the second is a complement of the first header. A headerinsertion means of the encoder inserts one of the first and secondheaders between each predetermined number of code words to therebyproduce frames.

Interference Cancelling

As described above, in an embodiment of the present invention a carriercomponent of a magnetic field signal (e.g., signal 104 transmitted bytransmit device 102) has a carrier component frequency that issubstantially equal to an integer multiple of 300 Hz. This guaranteesthat the carrier component frequency is substantially equal to aninteger multiple of both 50 Hz and 60 Hz. This carrier componentfrequency is selected to provide a maximum amount of spectral room forinformation sidebands. However, as mentioned above, using a carriercomponent frequency that is substantially equal to an integer multipleof 300 Hz will significantly increase the probability that magneticfield interference will interference with magnetic field signal strengthmeasurements when power lines are within the vicinity of transmit device102 and/or receive device 110, unless steps are taken to avoid suchinterference. As explained above, location determinations can be derivedbased on magnetic field strength measurements of the carrier component,for example, using the process described above in connection with FIGS.2A, 2A and 3. As also explained above, magnetic field interference(e.g., produced by power lines) can adversely affect the reliability andaccuracy of location determines, and the stability of displays showingthe location. The following embodiments of the present invention aredirected to reducing (and preferably cancelling) such interference, tothereby improve the reliability and accuracy of location determinations,and the stability of a display of the location (e.g., on display 730).

Prior to explaining these embodiments of the present invention,additional details of how magnetic field interference can affectmagnetic field strength measurements of a carrier component areprovided. As explained in the discussion of FIGS. 7 and 8, the outputsignal provided from DSP 708 to system computer 720 is an near DC signalwhen the magnetic field signal received at antenna array 702 does notinclude interference. The near DC signal represents the magnitude of thecarrier component of the magnetic field signal.

An exemplary near DC signal is shown in FIG. 11A. System computer 720determines a location of the underground transmit device 102 based onthe magnitude of the near DC signal. However, in the presence ofreceived interference, the signal provided from DSP 708 (to systemcomputer 720) tends to be amplitude modulated by the interference suchthat a beat frequency results from the interference. This will adverselyaffect the reliability, accuracy, and stability of locationdeterminations.

As mentioned above, power lines of power distribution networks produceharmonica interference signals at regular 50 Hz or 60 Hz intervals fromtheir fundamental frequency through to well above 10 kHz. Moreprecisely, the harmonic intervals are 50 Hz ±0.1 Hz or 60 Hz ±0.1 Hz.For example, if harmonic intervals caused by power lines in an area(e.g., within the United States) are at 60.0035 Hz intervals, then the140^(th) harmonic is at approximately 8400.5 Hz. The beat frequency of areceived signal including both an 8400 Hz carrier component frequencyand an 8400.5 Hz interference signal, after being shifted down (i.e.,mixed down) by 8400 Hz, is shown in FIG. 11C. More specifically, FIG.11A is a time domain illustration of an 8400 Hz carrier componentfrequency shifted down to near DC. FIG. 11B is a time domainillustration of a 8400.5 Hz interference signal shifted down by 8400 Hzto a signal having a frequency of approximately 0.5 Hz. FIG. 11C is atime domain illustration of a combined signal (including the 8400 Hzcarrier and the 8400.5 Hz interferer) after being shifted down by 8400Hz. The combined signal of FIG. 11C (e.g., the signal received byreceive device 110) is shown as having a beat frequency of 0.5 Hz. Inother words, after down conversion, the combined signal has a cycleperiod of one second.

It is noted, if a magnetic interference frequency is offset from thecarrier component frequency such that it falls outside of an operatingbandwidth of signal strength measurement system (e.g., if offset fromthe carrier component frequency by more than 0.5 Hz, assuming anexemplary operating bandwidth of 0.5 Hz), after being shifted down tobaseband, then the filters of digital signal processor 708 should filterout the magnetic interference. Accordingly, magnetic interferenceoutside the operating bandwidth of the system should not adverselyaffect the magnetic field strength measurements at antenna array 702 ofreceive device 110. However, if the magnetic field interference iswithin, for example, 0.5 Hz of the carrier signal at baseband, thenlocation determinations (of solenoid 210, and thus boring tool 430) canbe adversely affected because the magnitude of the carrier signal isaltered by magnetic interference, as shown in FIG. 11C. The reason thisoccurs is that the magnetic interference falls within the bandwidth ofthe magnetic field strength measurement processing. Accordingly, thereis a need to reduce (and preferably cancel) magnetic field interferencethat is within such a close proximity (e.g., within 0.5 Hz) of thecarrier component frequency.

The specific embodiments of the present invention that are directed toreducing (and preferably cancelling) the magnetic field interferencethat is within such a close proximity to the carrier component frequency(i.e., within the operating bandwidth of the field strength measuringsystem) shall now be described. These embodiments are explained withreference to the flowchart of FIG. 12, which outlines a method 1200 forreducing magnetic interference.

At a first step 1202, a plurality of samples that are representative ofa detected carrier component magnetic field strength are produced. Thisstep can be performed, for example, by A/D 706 and DSP 708. For example,these samples can be samples of the signal of FIG. 11C, which has justbeen discussed above.

At a next step 1204, a plurality of n different moving averages of theplurality of samples are produced, where n is at least two. Each movingaverage is produced by averaging successive sets of the samples, tothereby produce successive average values corresponding to thesuccessive sets of samples from step 1202. Thus, each moving averageincludes a plurality of average values. Each of the different movingaverages is a moving average of a different number of the plurality ofsamples (i.e., the set size associated with a first moving average isdifferent from a set size associated with a second moving average). Morespecifically, each of the different moving averages has a differentmoving average length. Each moving average length corresponds to atime-span over which the samples within the respective moving averageextend. The different moving averages can be produced in parallel, asshown in FIG. 12. For example: at a step 1204 a, a 1^(st) moving averageis produced; at a step 1204 b a 2^(nd) moving average is produced; . . .and at a step 1204 n, an n^(th) moving average is produced. In anotherembodiment, rather than producing the moving averages in parallel, themoving averages are produced serially.

At a next step 1206, a respective quality metric is determined for eachmoving average produced at step 1204. For example, at a step 1206 a, a1^(st) quality metric is produced for the moving average produced atstep 1204 a; at a step 1206 b a 2^(nd) quality metric is produced; . . .and at a step 1204 n, an n^(th) quality metric is produced. Each qualitymetric can be an amplitude variance (σ² ) of the corresponding movingaverage. A well known equation for variance is:

$\sigma^{2} = \frac{\sum\limits_{i = 1}^{N}\left( {x_{i} - \mu} \right)^{2}}{N}$In this example,

-   -   σ² represents the amplitude variance of a moving average,    -   x₁ represents the amplitude of one average value in the moving        average,    -   N represents the number of average values in the moving average,        and    -   μ represent the mean (i.e., average) amplitude of the average        values in the moving average.        The above equation determines biased amplitude variance. Other        types of amplitude variance that can be used include unbiased        amplitude variance (where the denominator is N−1) and absolute        variance. Those of skill in the art will appreciate that        additional measures of variance can also be used, or measures of        standard deviation can be used.

In another embodiment, each quality metric can be a difference betweenthe minimum average value and maximum average value of a respectivemoving average. This can be accomplished by determining a respectivemaximum and minimum of each of the moving averages. A difference betweenthe maximum and minimum of each of the moving averages is thendetermined. In still another embodiment, each quality metric can be theratio of the minimum to maximum of a respective moving average.

At a next step 1208, a preferred moving average (that is, a preferredone of the plurality of moving averages produced at step 1204) isselected based on the quality metrics determined at step 1206. Forexample, if the quality metrics determined at step 1206 are variances,then the moving average corresponding to the lowest variance is selectedat step 1208. If the quality metrics determined at step 1206 are thedifferences between maximums and minimums of each moving average, thenthe moving average corresponding to the lowest difference is selected atstep 1208. Similarly, if the quality metrics determined at step 1206 areratios of the minimums and maximums of each moving average, then themoving average corresponding to the lowest ratio is selected at step1208.

At a next step 1210, further signal processing is performed using thepreferred moving average. For example, the preferred moving average canbe used to monitor the location of an underground boring tool.

As mentioned above, each of the moving averages corresponds to arespective moving average length that is representative of the timeinterval during which the samples within the moving average wereproduced. Each moving average length is different from the other movingaverage length(s). In an embodiment of the present invention, additionalmoving averages of additional samples of the magnetic field signal areproduced, and used for further signal processing (e.g., used to monitorthe location of an underground boring tool). These additional movingaverages have a moving average length equal to the length of the movingaverage selected at step 1208 (i.e., a preferred moving average length).This can occur following step 1210. Alternatively, this can occur inplace of step 1210. The additional moving averages having the preferredmoving average length are then used for further signal processing (e.g.,to determine a location of an underground boring tool).

The different moving average lengths should be within (i.e., correspondto) a range of expected interference cycle periods that may adverselyaffect the carrier component of transmitted magnetic field signal 104because the interference cycle periods fall within the operatingbandwidth of the measurement system. More specifically, the differentmoving average lengths should collectively span the range of expectedinterference cycle periods that can affect measurements of the carriercomponent. Another way to look at this is that the moving averagelengths should collectively span an operating bandwidth of the signalmeasurement system.

As mentioned above, an interfering harmonic that is offset from thefrequency of the carrier component by more than 0.5 Hz should befiltered out by filters of stages 704 and/or 802 because it is withinthe operating bandwidth of the system. In such a system, only thoseinterfering harmonics within, for example, 0.5 Hz of the carriercomponent frequency of magnetic field signal 104 will adversely affectcarrier component measurements. Therefore, in the example system havinga 0.5 Hz operating bandwidth, the different moving average lengthsshould collectively span a range in time corresponding to a range ininterference frequencies beginning at a frequency near 0 Hz and endingat the frequency of 0.5 Hz.

For example, it can be appreciated that a moving average length of 0.5seconds or 1 second applied to the cyclically repeating corrupted signalof FIG. 11C, will result in a moving average including average valuessubstantially equal to the magnitude of the desired signal in FIG. 11A.Thus, an appropriately chosen moving average length reduces the effectsof an interfering harmonic (in this example, substantially cancels theeffects of the interfering harmonic). Accordingly, the moving averageselected at step 1208 can be thought of as a filtered signal.

In a preferred embodiment, the moving average lengths span betweenone-times and less than two-times the cycle length of expectedinterference. Thus, for the example of FIG. 11C, the moving averagelengths span between 2 seconds and 4 seconds (e.g., 2 seconds, 2.5seconds, 3.0 second and 3.5 seconds, respectively corresponding tointerference frequencies of 0.5 Hz, 0.4 Hz, 0.33 Hz and 0.26 Hz).However, the longest moving average length should be shorter than twicethe shortest moving average length. This is because the longer movingaverage length, of two moving average lengths that are both integermultiples of a cycle length of an interferer, will always be more stableand thus selected as the preferred moving average. This increases thelatency of the system without significantly improving performance. It isnoted that operating bandwidths a system can have operating bandwidthsother than 0.5 Hz, and thus, these embodiments of the present inventionare not limited to this operating bandwidth.

The samples from which the moving averages are derived can be producedwith a constant sample spacing (that is, time between samples). If thisis the case, each moving average length is defined by a product of thenumber of samples in the respective moving average and the samplespacing.

FIG. 13 is a function block diagram that can also be used to describedthe interference cancelling of the present invention. As shown, firstmoving averager 1302 a, second moving averager 1302 b, third movingaverager 1302 c and fourth moving averager 1302 d each receive (e.g.,from DSP 708) samples (for example, magnitude samples) that arerepresentative of magnetic field strength. Each moving averager outputsa respective moving average signal 1304 a, 1304 b, 1304 c and 1304 d,each moving average signal including a plurality of average values.Quality metric generators (Q.M.G.s) 1306 a, 1306 b, 1306 c and 1306 dproduce respective quality metric outputs 1308 a, 1308 b, 1308 c and1308 d that can be measures of variance, difference between maximum andminimums, or ratios between maximums and minimums, as discussed above. Aselector 1312 selects a preferred moving average based on the qualitymetric outputs 1308 a, 1308 b, 1308 c and 1308 d. Selector 1312 thendirects a switch 1310 to pass forward a preferred one of moving averagesignals 1304 a, 1304 b, 1304 c and 1304 d to be used for further signalprocessing (e.g., to be used by system computer 720 to produce locationdeterminations). The boundaries of the functional building blocks shownin FIG. 13 have been arbitrarily defined for the convenience of thedescription. Alternate boundaries can be defined so long as thespecified functions and relationships thereof are appropriatelyperformed. For example, various function can be combined into one block(e.g., selector 1312 and switch 1310 can be combined).

Assume the samples received by moving averagers 1302 a, 1302 b, 1302 cand 1302 d have a rate of 30 samples per second. If moving averager 1302a produces a moving average of 60 samples, then its moving averagelength is 2 seconds. Similarly, if moving averager 1302 b produces amoving average of 75 samples, then its moving average length is 2.5seconds. Similarly, if moving averager 1302 c produces a moving averageof 90 samples, then its moving average length is 3.0 seconds. Similarly,if moving averager 1302 d produces a moving average of 105 samples, thenits moving average length is 3.5 seconds.

Referring to FIG. 7, the interference cancelling embodiments of thepresent invention can be performed between DSP 708 and system computer720. Alternatively, these embodiments can be performed within DSP 708and/or system computer 720. The features of these embodiments can beperformed by hardware, entirely within software, or by a combination ofhardware and software.

CONCLUSION

The previous description of the preferred embodiments is provided toenable any person skilled in the art to make or use the embodiments ofthe present invention. While the invention has been particularly shownand described with reference to preferred embodiments thereof, it willbe understood by those skilled in the art that various changes in formand details may be made therein without departing from the spirit andscope of the invention. For example, some embodiments of the presentinvention have been described as methods with reference to flow charts.The present invention is also directed to systems that perform thefeatures discussed above. For example, the present invention is alsodirected to hardware and/or software that performs specific features ofthe present invention. Additionally, the present invention is alsodirected to a transmit device and receive devices that include hardwareand/or software for performing such features. Other embodiments of thepresent invention are directed to transmitted magnetic field signalsand/or systems for transmitting such signals.

Embodiments of the present invention have also been described above withthe aid of functional building blocks illustrating the performance ofspecified functions and relationships thereof. The boundaries of thesefunctional building blocks have often been arbitrarily defined hereinfor the convenience of the description. Alternate boundaries can bedefined so long as the specified functions and relationships thereof areappropriately performed. Any such alternate boundaries are thus withinthe scope and spirit of the claimed invention. One skilled in the artwill recognize that these functional building blocks can be implementedby discrete components, application specific integrated circuits,processors executing appropriate software and the like or anycombination thereof. Thus, the breadth and scope of the presentinvention should not be limited by the above-described exemplaryembodiments, but should be defined in accordance with the followingclaims and their equivalents.

1. A method for generating a magnetic field signal useable for locatingan underground object comprising: generating a reference signal having areference signal frequency substantially equal to the integer multipleof 300 Hz; receiving an information signal; encoding the informationsignal to produce an encoded information signal; modulating thereference signal with the encoded information signal at a predeterminedbit rate to produce the drive signal, the drive signal including thecarrier component and at least one information sideband includingsideband energy, the encoding and bit rate causing a substantial portionof the sideband energy to be contained between the carrier componentfrequency and a frequency spaced 50 Hz from the carrier componentfrequency; and driving a transponder with the drive signal to generatethe magnetic field signal, the magnetic field signal having a magneticfield carrier component equal to the carrier component frequency.
 2. Themethod of claim 1, wherein driving a transponder comprises driving thetransponder with the drive signal to generate the magnetic field signal,the magnetic field signal having a magnetic field signal carriercomponent equal to the carrier component frequency and at least onemagnetic field signal information sideband including magnetic fieldsignal sideband energy, a substantial portion of the magnetic fieldsignal sideband energy contained between the carrier component frequencyand the frequency spaced 50 Hz from the carrier component frequency. 3.The method of claim 2, wherein modulating the reference signal comprisesamplitude modulating the reference signal with the encoded informationsignal at a bit rate between 80 bits per second (pbs) and 50 bps toproduce the drive signal.
 4. The method of claim 3, wherein modulatingthe reference signal comprises amplitude modulating the reference signalwith the encoded information signal at a bit rate of 75 bits per secondto produce the drive signal.
 5. A method for generating a magnetic filedsignal useable for locating an underground object comprising: generatinga reference signal having a reference signal frequency substantiallyequal to the integer multiple of 300 Hz; receiving an informationsignal; encoding the information signal to produce an encodedinformation signal; modulating the reference signal with the encodedinformation signal at a predetermined bit rate to produce the drivesignal, the drive signal including the carrier component and a lowerinformation sideband and an upper information sideband, each of thelower and upper information sideband including respective sidebandenergy, the encoding and bit rate causing (i.) a substantial portion ofthe lower sideband energy to be contained between the carrier componentfrequency and a frequency spaced 50 Hz below the carrier componentfrequency, and (ii.) a substantial portion of the upper sideband energyto be contained between the carrier component frequency and a frequencyspaced 50 Hz above the carrier component frequency; and driving atransponder with the drive signal to generate the magnetic filed signal,the magnetic field signal having a magnetic field carrier componentequal to the carrier component frequency.
 6. The method of claim 5,wherein driving a transponder comprises driving the transponder with thedrive signal to generate the magnetic field signal, the magnetic fieldsignal having a magnetic field signal carrier component equal to thecarrier component frequency, a lower magnetic field signal informationsideband, and an upper magnetic field signal information sideband,wherein the lower magnetic field signal information sideband includesmagnetic field signal lower sideband energy, a substantial portion ofthe magnetic field signal lower sideband energy contained between thecarrier component frequency and the frequency spaced 50 Hz below thecarrier component frequency, and wherein the upper magnetic field signalinformation sideband includes magnetic field signal upper sidebandenergy, a substantial portion of the magnetic field signal uppersideband energy contained between the carrier component frequency andthe frequency spaced 50 Hz above the carrier component frequency.
 7. Themethod of claim 6, wherein modulating the reference signal comprisesamplitude modulating the reference signal with the encoded informationsignal at a bit rate between 80 bits per second (pbs) and 50 bps toproduce the drive signal.
 8. The method of claim 7, wherein modulatingthe reference signal comprises amplitude modulating the reference signalwith the encoded information signal at a bit rate of 75 bits per secondto produce the drive signal.
 9. A system for generating a magnetic filedsignal usable for locating an underground object comprising: a subsystemto produce a drive signal including a carrier component frequencysubstantially equal to an integer multiple of 300 Hz; a transponder togenerate the magnetic field signal when driven by the drive signal, thecarrier component equal to the carrier component frequency; wherein thesubsystem comprises: a reference signal generator to produce a referencesignal having a reference signal frequency substantially equal to theinteger multiple of 300 Hz; at least one sensor to produce aninformation signal; an encoder to encode the information signal toproduce an encoded information signal; a modulator to modulate thereference signal with the encoded information signal at a predeterminedbit rate to produce the drive signal, the drive signal including thecarrier component and at least one information sideband includingsideband energy, the encoder and bit rate causing a substantial portionof the sideband energy to be contained between the carrier componentfrequency and a frequency spaced 50 Hz from the carrier componentfrequency; and a transponder to generate the magnetic filed signal whendriven by the drive signal, the magnetic filed signal having a magneticfiled carrier component equal to the carrier component frequency. 10.The system of claim 9, wherein the magnetic field signal also has atleast one magnetic field signal information sideband including magneticfield signal sideband energy, a substantial portion of the magneticfield signal sideband energy contained between the carrier componentfrequency and the frequency spaced 50 Hz from the carrier componentfrequency.
 11. The system of claim 10, wherein the modulator amplitudemodulates the reference signal with the encoded information signal at abit rate between 80 bits per second (pbs) and 50 bps to produce thedrive signal.
 12. The system of claim 10, wherein the modulatoramplitude modulates the reference signal with the encoded informationsignal at a bit rate of 75 bits per second to produce the drive signal.13. The system of claim 9, wherein the at least one information sidebandproduced by the modulator comprises a lower information sideband and anupper information sideband, each of the lower and upper informationsideband including respective sideband energy, the encoder and bit ratecausing (i.) a substantial portion of the lower sideband energy to becontained between the carrier component frequency and a frequency spaced50 Hz below the carrier component frequency, and (ii.) a substantialportion of the upper sideband energy to be contained between the carriercomponent frequency and a frequency spaced 50 Hz above the carriercomponent frequency.
 14. The system of claim 13, wherein the magneticfield signal generated by the transponder, when driven by the drivesignal, includes a magnetic field signal carrier component equal to thecarrier component frequency, a lower magnetic field signal informationsideband, and an upper magnetic field signal information sideband, thelower magnetic field signal information sideband including magneticfield signal lower sideband energy, a substantial portion of themagnetic field signal lower sideband energy contained between thecarrier component frequency and the frequency spaced 50 Hz below thecarrier component frequency, and the upper magnetic field signalinformation sideband including magnetic field signal upper sidebandenergy, a substantial portion of the magnetic field signal uppersideband energy contained between the carrier component frequency andthe frequency spaced 50 Hz above the carrier component frequency. 15.The system of claim 13, wherein the modulator amplitude modulates thereference signal with the encoded information signal at a bit ratebetween 80 bits per second (pbs) and 50 bps to produce the drive signal.16. The system of claim 13, wherein the modulator amplitude modulatesthe reference signal with the encoded information signal at a bit rateof 75 bits per second to produce the drive signal.